Eevee is a Generation 1 Pokémon, and FireRed is a remake of the original Generation 1 games (Red and Blue). Eevee was introduced in Generation 2 (Gold and Silver).
To get Eevee, you'll need to play a game from Generation 2 or later, such as:
* Pokémon Gold, Silver, Crystal
* Pokémon Ruby, Sapphire, Emerald
* Pokémon Diamond, Pearl, Platinum
* Pokémon Black, White, Black 2, White 2
* Pokémon X, Y, Omega Ruby, Alpha Sapphire
* Pokémon Sun, Moon, Ultra Sun, Ultra Moon
* Pokémon Let's Go Pikachu/Eevee
* Pokémon Sword and Shield
* Pokémon Brilliant Diamond and Shining Pearl
* Pokémon Legends: Arceus
* Pokémon Scarlet and Violet
You can also potentially trade for an Eevee with someone who has it from a later generation game.